package org.apache.lucene.search.function;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.search.FieldCache;

/**
 * Expert: A base class for ValueSource implementations that retrieve values for
 * a single field from the
 * {@link org.apache.lucene.search.FieldCache FieldCache}.
 * <p>
 * Fields used herein nust be indexed (doesn't matter if these fields are stored
 * or not).
 * <p>
 * It is assumed that each such indexed field is untokenized, or at least has a
 * single token in a document. For documents with multiple tokens of the same
 * field, behavior is undefined (It is likely that current code would use the
 * value of one of these tokens, but this is not guaranteed).
 * <p>
 * Document with no tokens in this field are assigned the <code>Zero</code>
 * value.
 * 
 * <p>
 * <font color="#FF0000"> WARNING: The status of the <b>search.function</b>
 * package is experimental. The APIs introduced here might change in the future
 * and will not be supported anymore in such a case.</font>
 * 
 * @author yonik
 */
public abstract class FieldCacheSource extends ValueSource {
	private String field;

	/**
	 * Create a cached field source for the input field.
	 */
	public FieldCacheSource(String field) {
		this.field = field;
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see org.apache.lucene.search.function.ValueSource#getValues(org.apache.lucene.index.IndexReader)
	 */
	public final DocValues getValues(IndexReader reader) throws IOException {
		return getCachedFieldValues(FieldCache.DEFAULT, field, reader);
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see org.apache.lucene.search.function.ValueSource#description()
	 */
	public String description() {
		return field;
	}

	/**
	 * Return cached DocValues for input field and reader.
	 * 
	 * @param cache
	 *            FieldCache so that values of a field are loaded once per
	 *            reader (RAM allowing)
	 * @param field
	 *            Field for which values are required.
	 * @see ValueSource
	 */
	public abstract DocValues getCachedFieldValues(FieldCache cache,
			String field, IndexReader reader) throws IOException;

	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Object#equals(java.lang.Object)
	 */
	public final boolean equals(Object o) {
		if (!(o instanceof FieldCacheSource)) {
			return false;
		}
		FieldCacheSource other = (FieldCacheSource) o;
		return this.field.equals(other.field) && cachedFieldSourceEquals(other);
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Object#hashCode()
	 */
	public final int hashCode() {
		return field.hashCode() + cachedFieldSourceHashCode();
	}

	/**
	 * Check if equals to another {@link FieldCacheSource}, already knowing
	 * that cache and field are equal.
	 * 
	 * @see Object#equals(java.lang.Object)
	 */
	public abstract boolean cachedFieldSourceEquals(FieldCacheSource other);

	/**
	 * Return a hash code of a {@link FieldCacheSource}, without the hash-codes
	 * of the field and the cache (those are taken care of elsewhere).
	 * 
	 * @see Object#hashCode()
	 */
	public abstract int cachedFieldSourceHashCode();
}
